Currently, mobile telephones have been found in the fields that require more bandwidth such as access to WWW and videophone in addition to audio. There is increasing demand for bandwidth. In response to this, so-called third generation schemes such as W-CDMA (Wideband Code Division Multiple Access) and MC-CDMA (Multicarrier CDMA) schemes are being introduced. Further, MBMS (Mobile Broadcast/Multicast Service) providing a multicast service are being standardized in 3GPP (3rd Generation Partnership Project).
FIG. 7 shows a configuration of a W-CDMA wireless access network according to the prior art. This configuration is disclosed, for example, in Japanese Patent Laid-Open No. 2003-111148. Specifications for MBMS are described in 3GPP TS22.146, TS23.846, TS25.346, TS25.992.
In FIG. 7, BM-SC 701 (Broadcast/Multicast Service Controller) is a service control apparatus for controlling the MBMS.
GGSN 702 (Gateway GPRS Support Node) is a GPRS gateway apparatus for relaying a GPRS packet. SGSN 703 (Serving GPRS Suppose Node) controls GPRS. A radio network gateway apparatus 708 is a gateway between a core network and radio access network control apparatus 704a, 704b and relays a data packet between the SGSN 703 and the radio access network control apparatus 704a, 704b. The GGSN 702, the SGSN 703, the radio network gateway apparatus 708 and the radio access network control apparatus 704 relay a data packet such as MBMS. These are collectively referred to herein as a “data packet gateway apparatus”.
The first and the second radio access network control apparatus 704a, 704b control respective wireless access networks. In the following description, the first and second radio access network control apparatus 704a, 704b are collectively referred to as a radio access network control apparatus 704. The radio access network control apparatus may be abbreviated by RNC. First and second base stations 705a, 705b are base stations for transforming a signal in a wired access network and a wireless data and transmitting and receiving the signal and the data. In the following description, the first and second base stations 705a, 705b are collectively referred to as a base station 705. The terminal 706 performs wireless communication. Cells A, B indicate ranges in which signals from the first and second base stations 705a, 705b can arrive respectively, that is, the first and second base stations 705a, 705b can communicate with the terminal 706 respectively.
MBMS is a service for transmitting to the terminal via the GGSN 702, the SGSN 703 and the wireless access network in one-way direction a packet inputted by a content provider or the like from another IP network to the BM-SC 701. In order to reduce traffic flowing through the core network or the wireless access network, a packet is not transmitted from SGSN 703 which is not in RA (Routing Area) to the radio access network control apparatus 704.
In FIG. 7, it is assumed that the radio access network control apparatus 704 and the base station 705 are integrally deployed and one radio access network control apparatus 704 is deployed for each cell. Typically, in wireless access networks, tens of base stations are deployed under a radio access network control apparatus. Therefore, by reducing traffic at an interface between the radio access network control apparatus and the base stations, communication cost is expected to be effectively reduced.
However, whether or not the MBMS can transmit a packet is determined for each radio access network control apparatus 704. Therefore, if one of a plurality of base stations 705 connected to the radio access network control apparatus 704 has a terminal 706 receiving the MBMS, an MBMS packet is transmitted to a plurality of base station connected to the radio access network control apparatus even when those base stations 705 do not have any terminal 706 receiving the MBMS. This transmitted MBMS packet is not used in an actual service.
By miniaturizing the radio access network control apparatus 704 and locating the radio access network control apparatus 704 and the base station 705 at the same location as shown in FIG. 7, it is expected that transmission of an MBMS packet to unneeded base stations 705 is prevented and traffic in the wireless access network is reduced. However, if the radio access network control apparatus 704 is simply miniaturized, the range, where radio access network control apparatus 704 can communicate with the terminal, is reduced. Therefore, when the terminal 706 is moved, the radio access network control apparatus 704 with which the terminal 706 communicates is frequently switched. As shown in FIG. 7, in order to reduce the impact of miniaturization of the radio access network control apparatus 704 to a node in the core network such as the SGSN 703, the radio network gateway apparatus 708 is deployed. When relaying a packet transmission or switching the radio access network control apparatus 704 communicating with the terminal 706, the radio network gateway apparatus 708 operates similar to the SGSN 703, thereby reducing the switching frequency. In the following operation, the process of the radio network gateway apparatus 708 may alternatively be performed by the SGSN 703.
FIG. 8 shows an example of a processing sequence when the terminal 706 is moved. The present explanation deals with packet transmission providing the MBMS in a wireless access network only. In FIG. 8, the BM-SC 701, GGSN 702 and the base station 704 are omitted from the overall configuration diagram in FIG. 7 for simplicity. In FIG. 8, outline arrows denote flows of packet data and shaded arrows denote flows of wireless data. In the prior art, the base station is only responsible for transmitting dedicated channel transformed by the radio access network control apparatus.
In the MBMS, although either multicast in which a plurality of nodes in a wireless access network can receive the same packet or unicast in which a packet is individually received for each node can be selected as a transport layer in the wireless access network, the present invention assumes that unicast is selected.
In FIG. 8, the terminal 706 first establishes a dedicated channel with the first radio access network control apparatus 704a. In this case, the first radio access network control apparatus 704a is a Serving RNC for controlling wireless communication such as RRC in the terminal 706. This dedicated channel transmits data from the SGSN 703 to the terminal 706 via the radio network gateway apparatus 708 and the first radio access network control apparatus 704a. A data which is transmitted toward the terminal 706 is transmitted as a packet data up to the first radio access network control apparatus 704a. The data is transmitted as a wireless data from the first radio access network control apparatus 704a to the terminal 706. The packet data is assumed to be a data transmitted as GTP (GPRS Tunneling Protocol) packets. The wireless data is assumed to be a logical channel defined at MAC layer such as DTCH (Dedicated Traffic Channel) or MTCH (MBMS point-to-multipoint Traffic Channel) in a W-CDMA wireless access network. In 3GPP standards, a transport channel, a physical channel or the like generated from transforming a logical channel in the radio access network control apparatus 704 and the base station 705 is transmitted synchronously on a frame basis. The present invention is also applicable to the case where these are wireless data.
At event 802, the terminal 706 enters into the cell B and can communicate with both the first base station 705a and the second base station 705b. The terminal 706 outputs a power measurement report 803 including a report of a signal for the second radio access network control apparatus 704b to the first radio access network control apparatus 704a. 
On the other hand, the second radio access network control apparatus 704b also detects that the terminal 706 can communicate with the terminal 706 (although the first radio access network control apparatus 704a is operating as a Serving RNC). The radio access network control apparatus which can communicate with the terminal 706 but does not control the wireless communication is referred to as a Drift RNC. Since the terminal 706 receiving MBMS is under control, the second radio access network control apparatus 704b performs MBMS registration process 804 for adding itself to targets for transmitting MBMS (3GPP TS25.346: MBMS Registration procedure, hereinafter the Technical Specification (TS) or Technical Report (TR) number in 3GPP and the name of the message or procedure is noted within parenthesis) along with the radio network gateway apparatus 708. Further, the radio network gateway apparatus 708 performs an MBMS session initiation process 805 (3GPP TS25.346: MBMS Session Start) with respect to the second radio access network control apparatus 704b. This allows the radio network gateway apparatus 708 to transmit a packet such as a signal 806.
On the other hand, when the first radio access network control apparatus 704a receives the power measurement report 803 for the second base station 705b from the terminal 706, the first radio access network control apparatus 704a adds the cell B to Active Set indicating cells with which the terminal 706 communicates. In this case, a process similar to the communication using the dedicated channel is performed. The dedicated channel data processed at the first radio access network control apparatus 704a is transmitted using a communication channel between the first radio access network control apparatus 704a and the second radio access network control apparatus 704b and the transmitted data is transmitted to the terminal 706 via the second radio access network control apparatus 704b and the second base station 705b. 
First, in order to establish a dedicated channel of the first radio access network control apparatus 704a with the second radio access network control apparatus 704b, a radio link addition process 807 (3GPP TS25.423: Radio Link Addition procedure) is performed. This initiates transmitting the dedicated channel data from the first radio access network control apparatus 704a to the second radio access network control apparatus 704b as shown by a signal 808.
The second radio access network control apparatus 704b performs a process 809 for establishing a channel with the terminal 706 (3GPP TS25.331: Active Set Update procedure). The terminal 706 simultaneously communicates with the both the first and second base stations 705a, 705b. This reduces the possibility of disconnecting the communication due to a change in a circumstance of the wireless network.
Next, at event 811, the first radio access network control apparatus 704a primarily controlling the terminal 706 (operating as the Serving RNC) determines switching of the radio access network control apparatus (3GPP SRNS Relocation in UTRAN). First, the first radio access network control apparatus 704a out-puts an RNC switching preliminary notification 812 to the radio network gateway apparatus 708 (3GPP TS25.423: Relocation Required).
The radio network gateway apparatus 708 requests switching of the radio access network control apparatus from the first radio access network control apparatus 704a by an RNC switching request/response procedure 813. In response to this, the first radio access network control apparatus 704a outputs an RNC switching execution request 814 including the dedicated channel to be switched and context information such as the sequence number of a PDCP packet where switching timing occurs (3GPP TS25.423: Relocation Commit) to the target second radio access network control apparatus 704b. 
In response to the RNC switching execution request 814, the second radio access network control apparatus 704b notifies the radio network gateway apparatus 708 of a RNC switching detection notification 815 indicating detection of the switching execution request (3GPP TS25.413: RELOCATION DETECT) and performs switching of the dedicated channel. After switching, the second radio access network control apparatus 704b outputs the RNC switching completion notification 816 (3GPP TS25.413: RELOCATION COMPLETE) to the radio network gateway apparatus 708.